Document FR 2 841 191 describes an example of such a runner. The channel-section shape makes it possible to impart a good second moment of area, and good resistance to buckling to the moving runner rail which can have to support the entire weight of the seat and of the occupant of the seat. Such a runner is generally satisfactory. In order to increase the lifespan of such runners, it is desirable to increase their second moment of area. Such an increase in second moment of area could be achieved by increasing the thicknesses of the rails, which has a direct negative impact on the cost of manufacturing the runner.